> > > I agree that if I was designing a panel to work efficiently as an >> arch I would use much smaller radii. However, if I was designing a >> rib to work efficiently as a beam I wouldn't make it as wide as it is >> tall and I wouldn't have the same cross section over most of its >> length. >> Phil F > >And I agree that structural arches can be made of wood. But not with any >radii suitable for use in a piano soundboard. All of the examples you've >come up with have quite a significant radius. And they are hardly shaped >like piano ribs. Your examples notwithstanding, the structural arch >principle does not apply to the piano soundboard rib. Relative to its >length and radius, and especially the relatively small cross-sections at >the two ends of the rib, wood is simply too easily compressed. Even >longitudinally. > >This idea keeps coming up, mostly, I suppose, because its defenders are not >willing to actually try it out for themselves--preferring to believe >legend, tradition and marketing hype to real-world logic and proof. I prefer to think of it as exercising my own judgement based on whatever knowledge I have about structures. And that judgement says that some arch behavior seems feasible. That's not to say that I can't be wrong. I just want to see more physical proof before being convinced. In spite of what's often implied on this list, not every opinion is based on marketing hype. And I agree that a simple experiment done by me would be the best way of convincing me one way or the other. I don't think I'm unwilling, I just haven't done it. > The >next time you have a M&H rim sitting around with its soundboard removed, >force a typical crown into a rib and glue the thing in! Once the glue has >dried and/or cured take away whatever mechanism you used to force the crown >and see what happens. (Lacking a M&H with its soundboard removed you can >always cobble up a suitable test fixture on a really sturdy workbench. Feel >free to cheat and make sure the two end buttresses are completely >rigid--unlike the rim of a M&H which, even with its spider, still has >some--albeit slight--flex to it. Just make sure the rib is sized, shaped >and crowned like a real-world piano rib.) It should be quickly and plainly >obvious to even the most stubborn skeptic that a typical soundboard rib in >either the M&H rim or the test fixture is not capable of functioning as a >structural arch. To complete the experiment, assuming for some reason your >rib doesn't instantly collapse like mine did, you can put an appropriate >load on it and toss in a few months of compression-set...you'll redefine >the meaning of reverse-crown! Now this is more convincing. I'll give it a try when I have a free moment, which should be any month now. > >And the ribs we use are not as wide as they are tall nor do they have the >same cross-section over most of their length. They are (nearly) always >taller than they are wide and they taper out from the center. One of the >several advantages of the rib-crowned soundboard system is that you can >size, shape and form each rib to achieve most any variety of load-carrying >and acoustical results. > >Del Are they tapering out from the center in height or width? Care to share a picture? Phil Ford
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